The present invention relates to a method and an apparatus for generating digital signal changes when determining the amount of fuel dispensed from a fuel pump unit in accordance with the preamble to claims 1 and 8.
When measuring the volume of fuel dispensed from a fuel pump unit, it is common to let the fuel flow through a measuring means, in which the fluid flow, via movable pistons in a measuring chamber in the measuring means, causes a rotational movement. The rotational movement is then converted by a pulse-generating device, which emits electric pulses of a frequency which is proportional to the speed of the rotational movement. By means of these pulses, a calculating unit then controls a counter to indicate the volume dispensed.
An example of an apparatus of this kind for measuring the volume of a flowing fluid is disclosed in Applicant""s WO 88/08518. In this apparatus the movable pistons in the measuring chamber are connected to a rotatable shaft assembly. When the fluid flows through the measuring chamber, the pistons are caused to move and the shaft assembly is rotated. To the shaft assembly there is connected a frequency generator in the form of an apertured disc, which is adapted to rotate with the shaft assembly. A pulse-generating device is adapted to sense, in a contactless manner, the movement of the frequency generator and convert it into pulses via an optical transmitter and an optical receiver, which during rotation of the disc cooperate with the apertures in the disc. By means of this apparatus, pulses are generated, the number of which corresponds to the volume that has passed through the measuring chamber. the pulse-generating device, in turn, is connected to a calculating unit for calculating the volume that has flown through the measuring chamber depending on the of pulses emitted by the pulse generator. To the calculating unit there is, in turn, connected a counter for indicating the dispensed volume.
A general wish for high accuracy when determining an amount of fuel dispensed from a fuel pump unit is that a high resolution of the measured signals is desired, i.e. one wants to have a large number of pulses for each unit of fuel volume. An increased resolution of signals from the frequency generator implies, however, that higher demands are placed on both the transmission and reception of frequency signals. For instance, when using an optical transmitter and receiver in cooperation with an apertured rotating disc according to WO 88/08518, higher and higher demands are placed on the transmitter and the receiver, the more and smaller apertures that are formed in the disc
The requirement for high resolution is in opposition to the need for clear and reliable signal detection and wishes for small dimensions and simple constructions.
The use of optical transmitters and optical receivers has the drawback that the components are relatively energy-intensive, which places high demands on the equipment being explosion-proof.
A further drawback of the construction according to WO 88/08518 is the demand for an opening for the shaft of the frequency generator (disc) since high demands are placed on shaft seals in pumps for vehicle fuels.
Based on that stated above, the object of the present invention is to improve the prior-art technique for determining the amount of fuel dispensed from a fuel pump unit.
A special object of the invention is to permit a high signal resolution at the same time as the possibilities of clear and reliable signal detection are improved. In this context, it is also desirable to permit the use of components of small dimensions.
According to the invention, these and other objects that will appear from the following description have now been achieved by means of a method and an apparatus which are of the types described by way of introduction and which besides have the features stated in the characterising clauses of claims 1 and 8.
The inventive method thus generates a periodic analog signal having a first frequency corresponding to a fuel volume flow in the pump unit. The amplitude of the analog signal is detected. Based on the amplitude of the analog signal, signal changes are effected, having a second frequency which is dependent on and higher than the first frequency.
In the generation of the signal changes the amplitude of the analog signal is compared with a number of predetermined amplitude values, and a signal change is effected when a predetermined amplitude value has been reached.
As a result, the frequency of the analog signal can be held comparatively low. This results in good detectability since the number of periodic signal changes that must be accommodated, formed on or emitted by the frequency generator can be kept low. The signals can also be given a comparatively grater amplitude, which results in improved detectability. Lower demands can thus be placed on the equipment required for detection of the signal.
By a second higher frequency, which depends on the first frequency, being generated based on the amplitude of the analog signal, a high resolution is obtained for good accuracy when determining amounts of fuel.
In this way, it has been possible to satisfy the two opposed desiderata, simple detectability and high resolution, at the same time.
By periodic analog signal is meant a signal whose amplitude varies periodically. It is preferred for the analog signal to be continuous and have no sharp amplitude changes. Suitable analog signals are sinus signals.
The signal changes may consist of pulses or pulse edges. Besides the signal changes can be distributed on one or more signals and are preferably digital.
According to one aspect of the invention, one detects a plurality of amplitude values during each period of the analog signal.
A great advantage of the invention is that reliable and contactless detection of an analog signal can be carried out also under difficult conditions for signal transmission. For instance, one embodiment of the invention, in which the analog signal comprises magnetic field changes, is extremely well suited for contactless detection of the analog field through a liquid-tight measuring chamber without lead-ins, such as for shafts.
In a preferred embodiment of the invention, the amplitude of the analog signal is detected simultaneously in a first and second phase angle position, which are phase-shifted relative to each other. On the one hand, this results in the obtaining of information about the direction in which the signal is transmitted; in the case of a rotary frequency generator, information on the direction of rotation is thus obtained. Moreover, it is possible to use the possibility of detecting the analog signal in the angle position where the amplitude measurement gives the clearest reading of a correct frequency position, i.e. where the absolute derivative of the amplitude is at its maximum. Thus, improved detectability and improved reliability are achieved. It is particularly advantageous to carry out measurements in two phase angle positions which are relatively displaced by a quarter of a period (90xc2x0) since the absolute derivative of the signal in the one phase angle position has high values when the absolute derivative of the signal in the other phase angle position has low values and vice versa.
It is particularly preferable that the frequency for the signal changes is proportional to the frequency of the analog signal.
The apparatus according to the invention comprises a frequency generator, which is adapted to generate a periodic analog signal, preferably a sinus signal. In a preferred embodiment of the inventive apparatus, the frequency generator comprises a frequency disc, in whose circumference magnets are arranged. Besides, it is preferable that the frequency generator is arranged in the measuring chamber of the pump unit or on the fuel side of a fluid-tight barrier, such as the wall of a measuring chamber, and that the signal emitted by the frequency generator is detectable in the contactless manner outside the measuring chamber or on the other side of the fluid-tight barrier.
The signal of the frequency generator is detected by a pulse-generating device. The pulse-generating device comprises at least one sensor, which has the capacity of detecting amplitude changes in the analog signal. These amplitude changes are used by a signal-processing unit for generating preferably digital signal changes, which have a higher frequency than the analog signal.
In a preferred embodiment, the pulse-generating device comprises at least one analog sensor, which is adapted to supply a voltage that varies in response to magnetic field variations.